De-Mapping Technique with Provision of Probability Information Including a Priori Information

ABSTRACT

A technique for de-mapping a point in a constellation diagram into a bit sequence is presented. The de-mapping provides for each bit of the bit sequence an output value with a sign of the output value indicating a bit value and a magnitude of the output value indicating probability information in the form of a distance to a decision boundary in the constellation diagram. A method aspect of the technique presented herein comprises receiving a signal indicative of a constellation point, wherein the constellation point represents a bit sequence having a most significant bit and at least one next significant bit, deriving a first output value for the most significant bit based on a first decision boundary, receiving a priori information, and deriving a second output value for the next significant bit based on the first output value, the a priori information and a second decision boundary

TECHNICAL FIELD

The present disclosure generally relates to a technique for de-mapping apoint in a constellation diagram into a bit sequence, wherein for eachbit of the bit sequence reliability information is provided. Thetechnique may be implemented in the form of a method, a computer programproduct or an apparatus.

BACKGROUND

The 3^(rd) Generation Partnership Project (3GPP) and otherstandardization bodies have long been working on telecommunicationssystems that provide peak data rates of 100 Mbps and beyond. As one theexample, the 3GPP Long Term Evolution (LTE) standard can be mentioned.In LTE, high peak data rates are achieved by sophisticated mechanismssuch as link adaptation and Hybrid Automatic Retransmission Request(HARQ) schemes.

In short, link adaptation allows a base station to select modulation andcoding parameters individually per user terminal based on the currentchannel quality. In LTE, link adaption is supported by OrthogonalFrequency Division Multiple Access (OFDMA) in the downlink and SingleCarrier FDMA (SC-FDMA) in the uplink. An LTE OFDM downlink signalcomprises multiple orthogonal sub-carriers that can be addressed todifferent user terminals.

HARQ schemes enhance the acknowledgement, retransmission and time-outfeatures of conventional ARQ schemes with Forward Error Correction, FEC,coding (using, for example, so-called Turbo Codes) and with thetransmission of error detection information (such as Cyclic RedundancyCheck bits). HARQ schemes improve system throughput by combining, ratherthan discarding, information received via previous erroneoustransmission attempts with information received with a current attempt.

FIG. 1 illustrates a block diagram of components of an exemplary OFDMreceiver 10 that supports HARQ. The receiver 10 comprises a Fast FourierTransformer (FFT) 20, a frequency domain equalizer 30, a de-mapper 40and a channel decoder 50.

An OFDM time domain signal received via a channel is converted by theFFT 20 into the frequency domain to extract the orthogonal sub-carriers.Downstream of the FFT 20 the equalizer 30 individually compensates thechannel impact for each sub-carrier in the frequency domain. To this endeach sub-carrier is processed based on an estimated channel frequencyresponse. Then, the de-mapper 40 (also referred to as demodulator)reproduces the digital information contained in the output signal of theequalizer 30 by de-mapping points in a constellation diagram into bitsequences. This process is also referred as demodulation and reversesthe mapping, or modulation, performed on a transmitter side. The channeldecoder 50 decodes the output of the de-mapper 40 and generatessequences of decoded bits as shown in FIG. 1.

During OFDM signal generation a transmitter maps, or modulates, digitalinformation onto sub-carriers for transmission via the channel.Sub-carrier modulation is performed by varying on or both of a phase andan amplitude of a sub-carrier in accordance with the digital informationto be transmitted, giving rise to both In-phase (I) and Quadrature (Q)sub-carrier waves.

By way of modulation, binary data are grouped into sequences of i bitsthat constitute one symbol. Hence, each symbol corresponds to one of2^(i) possible points, and the total number of points is referred to asa constellation. A constellation can be represented in the form of aconstellation diagram in an I/Q plane, wherein the values of I and Q maybe interpreted as the real and imaginary parts, respectively.

Now returning to the receiver 10 of FIG. 1, the de-mapper 40 is incharge of de-mapping a point in the constellation diagram into theunderlying symbol, or bit sequence. A de-mapping scenario for anexemplary Quadrature Amplitude Modulation (QAM) constellation with 16points in the I/Q plane (“16-QAM”) and a resulting bit sequence b₃b₂b₁b₀having a length of i=4 is described in Ch. Axell and M. Brogsten,“Efficient WiMAX Receiver Implementation on a Programmable BasebandProcessor”, LiTH-ISY-EX-06/3858-SE, Linköping University (2006-10-12),section 7.5.1 (http://www.ep.liu.se/).

FIG. 2 illustrates the 16-QAM de-mapping procedure for the MostSignificant Bit (MSB) b₃ of the bit sequence b₃b₂b₁b₀=0000₂. Thisspecific bit sequence corresponds to constellation point [I, Q]=[1, 1]in the 16-QAM I/Q plane.

In FIG. 2, the output signal of the equalizer 30 is marked by “X”. Ascan be seen, the marking “X” does not fully coincide with theconstellation point [1, 1] due to channel variations and otherimperfections. For this reason a decision procedure is performed by thede-mapper 40 to identify a point in the constellation diagram thatcorresponds to the marking “X”. This decision procedure is based onrepeatedly applying decision boundaries in the constellation diagram.The exemplary decision boundary illustrated in FIG. 2 for MSB b₃ isselected to coincide with the Q axis (i.e., an axis defined by I=0). Ifthe received in-phase signal portion is larger than zero (I>0) thetransmitted symbol can be assumed to be located in the right half planein FIG. 2, where all symbols with MSB b₃=0 are located. Accordingly, theboundary decision illustrated in FIG. 2 results in b₃=0. Similarboundary decisions (but using other decision boundaries) are performedfor the remaining bits b₂, b₁, and b₀.

The decision procedure outlined above only provides the bit sequenceb₃b₂b₁b₀ with no additional information about the reliability, orprobability of correctness, of the individual decisions. In other words,the output of the de-mapper 40 provided to the decoder 50 does notpermit any conclusions about the closeness of the marking “X” in FIG. 2to the associated point [1, 1] in the constellation diagram. Evidently,the operation of the decoder 50 would benefit from such probabilityinformation. Therefore, Ch. Axell and M. Brogsten suggest adopting theso-called Ramesh algorithm that also provides probability informationduring the de-mapping process (see sections 7.5.2 and 7.5.3).

The Ramesh algorithm uses decision boundaries as explained above butproduces supplemental information about the probability of an individualdecision. Specifically, the Ramesh algorithm generates for each decision(i.e., each bit of the bit sequence) an output value in the form of asigned magnitude representative of extrinsic probability information.The sign of the output value is indicative of a bit value 1 or 0, andthe magnitude (i.e., the absolute value) is indicative of a distance tothe applied decision boundary.

The signed magnitude output by the Ramesh algorithm constitutesextrinsic probability information for the decoder 50. This probabilityinformation, which is sometimes also referred to as “soft bit”information, significantly improves the performance of an errorcorrection algorithm and other procedures implemented in the decoder 50.

SUMMARY

There is a need to further improve the performance of a de-mappingtechnique that is based on an algorithm of the Ramesh type.

According to one aspect, a method of de-mapping a point in aconstellation diagram into a bit sequence is presented, wherein thede-mapping provides for each bit of the bit sequence extrinsicprobability information in the form of an output value with a sign ofthe output value indicating a bit value and a magnitude of the outputvalue indicating probability information in the form of a distance to adecision boundary in a constellation diagram. The method comprisesreceiving a signal indicative of a constellation point, wherein theconstellation point represents a bit sequence having a most significantbit and at least one next significant bit, deriving a first output valuefor the most significant bit, receiving a priori information pertainingto the most significant bit, and deriving a second output value for thenext significant bit based on the first output value, the a prioriinformation and a decision boundary.

The a priori information may be information that is available before thede-mapping has been performed. The a priori information may be receivedfrom any process different from the de-mapping process (e.g., a decodingprocess). Extrinsic information may generally be considered as newinformation (e.g., that has not been input in the de-mapping process).The terms a priori information and extrinsic information may be used ona bit-wise granularity level in connection with the present disclosure.For example, the (extrinsic) second output value for the nextsignificant bit may be derived taking into account a priori informationpertaining to (e.g., generated for or from) the most significant bit.

Deriving the second output value may comprise modifying at least one ofthe first output value and the decision boundary based on the a prioriinformation. Such a modification may comprise shifting at least one ofthe first output value and the decision boundary by a distance definedby the a priori information. For example, deriving the second outputvalue may comprise a comparison of the modified first output value withthe decision boundary. In another example, a comparison of the firstoutput value with the modified decision boundary may take place.

In a similar manner, deriving the first output value may comprisecomparing the received signal, or a signal derived therefrom by thede-mapper, with another decision boundary. The other decision boundarymay be zero.

Generally, at least one of the decision boundaries may be parallel to anaxis of the constellation diagram. This includes that at least one ofthe decision boundaries coincides with an axis of the constellationdiagram.

In certain configurations, the methods or individual steps thereof maybe performed separately for a real part and imaginary part of thereceived signal. In such a case, at least one of the decision boundariesmay be represented by a point on an axis of the constellation diagram.Such a point may be constituted by zero.

The a priori information may stem from one or multiple processesperformed upstream or downstream of the de-mapping process. As anexample, the a priori information may be obtained from a decodingprocess (e.g., from a channel decoding process using a turbo or othercode). As a further example, the a priori information may be obtainedfrom a correlation (or de-correlation) of media frames (e.g., speech orvideo frames) included in the transmitted information. Still further,the a priori information may have been obtained from iterativeprocessing or from processing one or more previous transmissions thatcould not be successfully decoded (e.g., in connection with an HARQimplementation).

In one variant, the a priori information takes the form of extrinsicinformation obtained, for example, by a decoding process. The decodingprocess may be performed downstream of the de-mapping process.Specifically, the decoding process may decode a bit sequencerepresentative of the most significant bit and one or more nextsignificant bits provided by the de-mapping process. The decodingprocess may comprise or be part of an HARQ scheme, optionally with oneor both of FEC (de-)coding and the processing of error detectioninformation, such as CRC bits.

The extrinsic information obtained by the decoding process may containprobability information, or “soft bit” information, generated in thedecoding process. The probability information contained in the extrinsicinformation may take any form, e.g., a binary form or the form of asigned magnitude as provided by a Ramesh-type algorithm.

The first output value may take the form of a Log-Likelihood Ratio (LLR)value. Additionally, or as an alternative, the a priori information maytake the form of an LLR value.

One or more of the method steps may be repeated with respect to a stillfurther decision boundary pertaining to one or more further nextsignificant bits. As such, the method may comprise receiving further apriori information pertaining to the next significant bit (together withor separately from the a priori information used for determining thedecision boundary) and deriving a third output value for a further nextsignificant bit based on the second output value, the further a prioriinformation and the still further decision boundary. As said, thereceiving and deriving steps may be repeated for one or more additionalnext significant bits.

The received signal may be a complex value comprising a real part and animaginary part (e.g., an I-part and a Q-part in an I/Q plane). In such acase, the method, or individual steps thereof, may be performedseparately for the real part and the imaginary part of the receivedsignal. Specifically, at least the steps of deriving the first outputvalue and deriving the second output value (and, optionally, any thirdor higher order output value) can be performed separately for the realpart and the imaginary part.

The received signal may have been modulated based on any suitablemodulation, or mapping, scheme. As an example, QAM (e.g., 16-QAM,64-QAM, 256-QAM, etc.) can be mentioned here.

Further provided is a computer program product comprising program codeportions for performing the steps of any of the methods and methodaspects described herein when the computer program product is run on acomputing device. The computer program product may be stored on acomputer-readable recording medium, such as a semiconductor memory, aCD-ROM, or DVD. Furthermore, the computer program product may beprovided for a download via a network connection.

Also provided is a de-mapper for de-mapping a point in a constellationdiagram into a bit sequence, wherein the de-mapping provides for eachbit of the bit sequence extrinsic probability information in the form ofan output value with a sign of the output value indicating a bit valueand a magnitude of the output value indicating probability informationin the form of a distance to a decision boundary in the constellationdiagram. The de-mapper comprises a first interface configured to receivea modulation symbol indicative of a constellation point, wherein theconstellation point represents a bit sequence having a most significantbit and at least one next significant bit, a second interface configuredto receive a priori information pertaining to the most significant bit,and at least one of a processor or circuitry configured to derive afirst output value for the most significant bit, and to derive a secondoutput value for the next significant bit based on the first outputvalue, the a priori information and a decision boundary.

The de-mapper may be part of a stationary or mobile component. As anexample, the de-mapper may be incorporated into a base station.Alternatively, the de-mapper may be incorporated into a wirelessterminal. The wireless terminal may, in addition to the de-mapper,comprise a decoder coupled to the de-mapper and configured to providethe a priori information. The de-mapper and the decoder may beintegrated into an OFDM receiver of the wireless terminal. The OFDMreceiver may be configured to operate in accordance with the 3GPP LTEstandard.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and advantages of the technique presented herein willbecome apparent from the following description of exemplary embodimentsand the drawings, wherein:

FIG. 1 shows a block diagram of an exemplary receiver;

FIG. 2 schematically illustrates an I/Q plane for 16-QAM and thede-mapping of a MSB;

FIG. 3A shows a block diagram of a first receiver embodiment;

FIG. 3B shows a block diagram of a second receiver embodiment;

FIG. 3C shows a block diagram of a de-mapper embodiment;

FIGS. 4A, 4B schematically illustrate an exemplary decision processbased on a Ramesh-type de-mapping algorithm; and

FIGS. 5A, 5B schematically illustrate an embodiment of a decisionprocess based on a Ramesh-type de-mapping process.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description of preferred embodiments, for purposes ofexplanation and not limitation, specific details are set forth toprovide a thorough understanding of the present disclosure. It will beapparent to one skilled in the art that the present invention may bepractised in other embodiments that depart from these specific details.For example, while the embodiments will be described in connection witha specific Ramesh-type algorithm, it will be appreciated that thepresent disclosure may also be practised in connection with otherimplementations of such an algorithm. Moreover, while the embodimentswill primarily be described in the context of the 3GPP LTE standard, itwill be evident that the disclosure presented herein can also bepractised in connection with other communications technologies, such asWiMAX.

Those skilled in the art will further appreciate that the services,functions and steps explained herein may be implemented using softwarefunctioning in conjunction with a programmed microprocessor, one or moreApplication Specific Integrated Circuits (ASICs), one or more DigitalSignal Processors (DSPs) or a general purpose computer. It will also beappreciated that while the following embodiments will primarily bedescribed with reference to methods and apparatuses, the disclosureprovided herein may also be embodied in a computer program product aswell as in a system comprising a processor and a memory coupled to aprocessor, wherein the memory stores one or more programs that cause theprocessor to perform the services, functions and steps disclosed herein.

FIG. 3A shows a block diagram of a wireless terminal 100 thatincorporates a first receiver embodiment 110. The wireless terminal 100can be realized as a mobile telephone, smartphone, tablet computer ornotebook. The receiver 110 is realized to support OFDM and operates inaccordance with 3GPP LTE or any similar standard.

As illustrated in FIG. 3A, the receiver 110 comprises an FFT 120, afrequency domain equalizer 130, a de-mapper 140 as well as a channeldecoder 150. The FFT 120, equalizer 130, de-mapper 140 and channeldecoder 150 may generally be operated in a similar manner as thecorresponding components of the OFDM receiver 10 illustrated in FIG. 1.For this reason, a detailed discussion of the FFT 120, equalizer 130 andchannel de-coder 150 will be omitted, and only certain technical detailsof the de-mapper 140 will be described hereinafter.

The de-mapper 140 is generally adapted to receive from the equalizer 130a signal indicative of a point in a constellation diagram and to de-mapthe point in the constellation diagram into a bit sequence. Thatde-mapping may generally result in one or multiple signed magnitudes atthe output of the de-mapper 140. In other words, the de-mapper 140 maybe configured to perform a de-mapping algorithm of the Ramesh type.

In more detail, the de-mapping performed by the de-mapper 140 providesfor each bit of the bit sequence extrinsic probability information inthe form of an output value. A sign of that output value indicates a bitvalue, and its magnitude indicates probability information in the formof a distance to a decision boundary in the constellation diagram.

As highlighted in FIG. 3A, the de-mapper 140 comprises an input forreceiving a priori information. This a priori information may bereceived from various sources of information. In one example, that willbe discussed in more detail with reference to FIGS. 3B and 3C, the apriori information is generated by and received from the channel decoder150. In other embodiments, the a priori information may be received froma source of information different from the channel decoder 150 or mayeven locally be generated and, for example, buffered by the de-mapper140.

As an example, if the transmitted information included in the signalreceived by the wireless terminal 100 stems from a speech or videoencoder, the speech or video encoded bits are typically correlated fromone speech or video frame to the next speech or video frame. Thiscorrelation can be analyzed to create a priori information that is thenfed to the de-mapper 140 as shown in FIG. 3A.

As another example, the a priori information may have been derived onthe basis of iterative processing or from one or multiple previoustransmissions of the same or similar information. In an exemplary LTEHARQ scenario, the decoding of an initial transmission by the decoder150 may have failed. In such a case, LTE HARQ defines that the inputsignal of the decoder 150 for the failed transmission (i.e, theextrinsic probability information received by the decoder 150 for thefailed transmission from the de-mapper 140) should be buffered. Thebuffered extrinsic probability information will then be combined withthe extrinsic probability information of one or more re-transmissions toincrease the performance of the decoder 150.

However, the extrinsic information thus buffered for an initialtransmission at, for example, the channel decoder 150 may also be fedback to the de-mapper 140 in the form of a priori information to beexploited in connection with de-mapping a re-transmission. Of course, itwould also be possible to buffer the extrinsic probability informationof an initial transmission locally at the de-mapper 140 or in a separatebuffer component for increasing the de-mapping performance of one ormore re-transmissions may follow.

FIG. 3B illustrates another embodiment of the de-mapper 140 that isbased on the embodiment illustrated in FIG. 3A and the example in whichthe a priori information fed to the de-mapper 140 is received from thechannel decoder 150. In the following, the configuration, the input andoutput parameters, and the operation of the de-mapper 140 of FIG. 3Bwill be described in more detail.

FIG. 3C illustrates internal components of the de-mapper 140 of FIG. 3B.As shown in FIG. 3C, the de-mapper 140 comprises a first interface 142coupled to the equalizer 130, a Central Processing Unit (CPU) ordedicated circuitry 144 as well as another interface 145 coupled to thedecoder 150. The interface 142 is configured to receive a noise symbolobservation as well as a noise variance parameter from the equalizer 130and to forward same to the CPU or circuitry 144. The interface 146 isconfigured to receive from the channel decoder 150 a priori informationat least for the MSB and to forward the a priori information to the CPUor circuitry 144. In the present case, the a priori information receivedfrom the decoder 150 is extrinsic information. For example, suchextrinsic information can be derived from exploiting code (e.g. turbocode) properties. The interface 146 is further configured to provideextrinsic probability information to the decoder 150.

In the following, a general formulation for the un-modified Rameshalgorithm will be presented first. Lets be the received signal in theform of a noisy symbol observation as, for example, obtained whentransmitting information over a wireless point-to-point communicationlink under an Additive White Gaussian Noise, AWGN, condition, i.e.,

ŝ=s+υ

s ∈

is the complex transmit symbol, where

set represents a finite symbol constellation or alphabet (e.g., 16-QAMor 64-QAM). It is assumed that the relation between a specific bitsequence and neighboring symbols is defined in accordance with Graymapping, i.e., bit sequences associated with neighboring symbolsdistinguish by only a single bit.

υ∈

is zero mean Gaussian noise with variance σ² _(υ), i.e. υ˜CN(0, σ² _(υ))

In order to obtain the extrinsic probability information as requiredfor, for example, turbo decoding by the channel decoder 150, the noisysymbol has to be de-mapped by the de-mapper 140. As a practical lowcomplexity solution a modified Ramesh algorithm that is based on the onedescribed in the document by Ch. Axell and M. Brogsten can be applied bythe de-mapper 140.

Let e.g. g denote the bit index with respect to the bit sequence oflength G associated with the real or imaginary part of a receivedsymbol. Then the output value of the de-mapper 140 corresponding to theMSB (g=0) of the real part is given by

$= \frac{{4 \cdot}\left\{ \hat{s} \right\}}{\sigma_{v}^{2} \cdot \sqrt{M}}$with $M = {\frac{2}{3}\left( {2^{2G} - 1} \right)}$

The remaining bits can be derived by the recursive rule

$\begin{matrix}{L_{g,} = {\frac{2^{G - g + 2}}{\sigma_{v}^{2} \cdot M} - {{{sgn}\left( L_{{g - 1},} \right)} \cdot L_{{g - 1},}}}} \\{= {\frac{2^{G - g + 2}}{\sigma_{v}^{2} \cdot M} - {L_{{g - 1},}}}}\end{matrix}$

De-mapping of the imaginary part is equivalently dealt with.

It should be noted that the absolute operation in the above equation canbe understood as “hard bit” decision depending on the sign of theprevious level output value. As an example, FIG. 4A depicts the decisionprocess and decision tree for the extrinsic soft bit de-mapping of thereal part of 64-QAM modulated symbols.

With reference to FIG. 4A, the output value L₀ for the MSB is calculatedin step 402. For calculation of the output value for the nextsignificant bit L₁, a decision process is made in step 404 depending onthe sign of the output value L₀ obtained for the MSB. Depending on theresult of the decision process in step 404, the output value L1 for thenext significant bit is calculated either in step 406 or in step 414.Following the decision process as in step 404 for L₀, a similar decisionprocess for L₁ is performed in step 408 or step 416, to then calculate athird output value for the further next significant bit L₂ in either oneof steps 410, 412, 418 or 420.

As said, the decision processes in steps 404, 408 and 416 can beregarded as “hart bit” decisions as will now be discussed in detail withrespect to an exemplary 16-QAM scenario illustrated in FIG. 4B.

For determining the output value L₀ for the MSB, the observation in theI/Q plane is assessed with respect to a first decision boundary definedby the Q axis (I=0) in the I/Q plane. The corresponding output value L₀will, in accordance with the Ramesh algorithm, be a signed magnitude.The sign is indicative of whether the observation lies in the positivesign half plane or the negative sign half plane of the I/Q plane asshown in FIG. 4B. The magnitude, on the other hand, indicates thedistance to the decision boundary I=0 in the I/Q plane.

Once the output value L₀ for the MSB has been determined, a decisionprocess similar to step 404 in FIG. 4A is performed as illustrated onthe right-hand side of FIG. 4B for calculating the output value L₁ forthe next significant bit. The result of that decision process depends onwhether the observation lies in the positive sign half plane or thenegative sign half plane of the I/Q plane. In the exemplary scenarioillustrated in FIG. 4B, where the observation lies in the positive signhalf plane, a new decision boundary is then determined by a right shiftof the previous decision boundary and a sign flip. The output value forthe next significant bit L₁ is determined on the basis of the shifteddecision boundary and the sign-flipped output value L₀ for the MSB in asimilar manner as indicated in step 406 of FIG. 4A for 64-QAM.

It will be appreciated that the procedures illustrated in FIGS. 4A and4B will also be performed for the imaginary part of the observation in asimilar manner but using decision boundaries coinciding with or parallelto the I axis in the I/Q plane.

In the scenarios illustrated in FIGS. 4A and 4B, the output values L₁,L₂, . . . for one or more next significant bits following the MSB arecalculated in a similar manner as the output value L₀ of the MSB. It hasbeen found that the reliability of processing operations downstream ofthe de-mapper 140, such as of the channel decoder 150, will benefit whentaking into account a priori information upon calculating the outputvalues L₁, L₂, . . . for the next significant bits. To this end, thede-mapper 140 of FIG. 3B is configured to receive a priori informationL^(a priori) pertaining to the most significant bit and, optionally, oneor more next significant bits from the channel decoder 150.

The CPU or circuitry 144 of the de-mapper 140 is configured to derivethe output value L₀ for the MSB based on a first decision boundary, andone or more further output values for one or more next significant bitsbased on associated further decision boundaries, output values forpreceding bits and the a priori information L^(a priori). For example,the output value L₁ for the next significant bit following the MSB isderived based on the output value L₀ for the MSB, the a prioriinformation L₀ ^(a priori) received from the channel decoder 150 for theMSB, and a second decision boundary.

Generally, starting from the MSB, output values for the remaining bitsare calculated recursively by the two steps

-   -   Calculate output value (LLR) of actual bit    -   Determine the rule of the subsequent calculation by evaluating        the sign of the actual output value and a priori information        from the channel decoder 150

Revisiting FIG. 4A, apparently the MSB output value L₀ (associated withbit level g=0) is completely independent of any other bit decision,whereas the output values L₁, L₂, . . . associated with the remaining,less significant bits (g>0) recursively depend on the intrinsic bitdecisions of the more significant bits. It is thus proposed to take notonly into consideration the intrinsic but also a priori bit informationof the lower level for the decision process.

Let, for example, L_(g) ^(a priori) be the extrinsic LLR provided by thechannel decoder 150, where subscript g relates to the bit index of thebit sequence keyed by one symbol. To improve the criterion at thedecision steps of FIGS. 4A and 4B, it is suggested to sum up intrinsicoutput values and a priori information values and to use the sum termfor comparison against zero (instead of only using the intrinsicinformation as in FIGS. 4A and 4B). Hence, the recursive update rule ofthe conventional Ramesh algorithm mutates into

$L_{g,} = {\frac{2^{G - g + 2}}{\sigma_{v}^{2} \cdot M} - {{{sgn}\left( {L_{{g - 1},} + L_{{g - 1},}^{a - {priori}}} \right)} \cdot L_{{g - 1},}}}$

The decision tree of the modified approach suggested herein is depictedin FIG. 5A. Steps 502 to 520 illustrated in the decision tree of FIG. 5Agenerally correspond to the associated steps illustrated in FIG. 4A forthe conventional Ramesh algorithm. However, as indicated for decisionsteps 504, 508 and 516, the a priori information received from thechannel decoder 150 is additionally taken into account in the associateddecision processes.

Specifically, in decision step 504, the output value L₀ for the MSB ismodified based on the a priori information L₀ ^(a priori) before beingcompared with the decision boundary I=0. That is, the “intrinsic” outputvalue L₀ and the value of the associated “extrinsic” a prioriinformation L₀ ^(a priori) derived by the decoder 150 for the MSB aresummed up before the boundary decision (sign evaluation) in step 504. Ofcourse, instead of modifying the output value L₀ in step 504, based onthe a priori information L₀ ^(a priori), the associated decisionboundary could alternatively be modified (i.e., shifted) by the negativeamount of L₀ ^(a priori).

As shown in FIG. 5A, the further decision steps 508 and 516 are modifiedin a similar manner as the decision step 504. That is, the intrinsicoutput value L₁ for the next significant bit is modified in steps 508and 516 by an amount defined by the extrinsic a priori information L₁^(a priori) generated by the channel decoder 150 for that bit.

While the decision tree of FIG. 5A illustrated the modification of theRamesh algorithm presented herein for a 64-QAM scenario, the diagram ofFIG. 5B highlights the differences with respect to the conventionalRamesh algorithm in comparison to the 16-QAM diagrams of FIG. 4B. Asshown in the middle of FIG. 5B, the consideration of the a prioriinformation in the decision process can be visualized by an a priorishift of the decision boundary (see dashed arrow).

The shift of the decision boundary due to the consideration of theadditional a priori information can significantly affect the outcome ofthe decision process and the following calculation of the output valueL₁ of the next significant bit. While, in the scenario of FIG. 4B, thedecision resulted in a calculation of L₁ in accordance with the upperbranch, the decision process of FIG. 5B based on the additional a prioriinformation led to the lower calculation branch. As such, theconsideration of additional a priori information as illustrated in FIGS.5A and 5B may lead to opposite bit values compared to a scenario as inFIGS. 4A and 4B, in which the a priori information is not considered.

The suggested modification of the conventional Ramesh algorithm israther incomplex and causes only a very low increase of computationalcomplexity. The output values are by nature extrinsic such thatsubtraction of a priori LLRs is not necessary.

It is believed that many advantages of the present disclosure will befully understood from the description above, and it will be apparentthat various changes may be made in the form, construction andarrangement of the exemplary aspects thereof without departing from thescope of the invention, or without sacrificing all of its advantages.Because the invention can be varied in many ways, it will be recognizedthat the invention should be limited only by the scope of the claimsthat follow.

1-19. (canceled)
 20. A method of de-mapping a point in a constellationdiagram into a bit sequence, wherein the de-mapping provides, for eachbit of the bit sequence, extrinsic probability information in the formof an output value, with a sign of the output value indicating a bitvalue and a magnitude of the output value indicating probabilityinformation in the form of a distance to a decision boundary in theconstellation diagram, the method comprising: receiving a signalindicative of a constellation point, wherein the constellation pointrepresents a bit sequence having a most significant bit and at least onenext significant bit; deriving a first output value (L₀) for the mostsignificant bit; receiving a priori information (L₀ ^(a priori)) for themost significant bit; and deriving a second output value (L₁) for thenext significant bit based on the first output value (L₀), the a prioriinformation and a decision boundary.
 21. The method of claim 20, whereinderiving the second output value comprises modifying at least one of thefirst output value and the decision boundary based on the a prioriinformation.
 22. The method of claim 21, wherein modifying at least oneof the first output value and the decision boundary comprises shiftingat least one of the first output value and the decision boundary by adistance defined by the a priori information.
 23. The method of claim21, wherein deriving the second output value comprises one of acomparison of the modified first output value with the decision boundaryand a comparison of the first output value with the modified decisionboundary.
 24. The method of claim 20, wherein deriving the first outputvalue comprises comparing the received signal with another decisionboundary.
 25. The method of claim 24, wherein the other decisionboundary is zero.
 26. The method of claim 20, wherein at least one ofthe decision boundary and the other decision boundary is parallel to anaxis of the constellation diagram.
 27. The method of claim 20, whereinthe method is performed separately for a real part and an imaginary partof the received signal.
 28. The method of claim 20, wherein the a prioriinformation is obtained by a decoding process.
 29. The method of claim20, wherein the a priori information takes the form of extrinsicinformation.
 30. The method of claim 20, wherein the a prioriinformation is obtained for a re-transmission from a previoustransmission.
 31. The method of claim 20, wherein at least one of thefirst output value and the a priori information takes the form of alog-likelihood ratio value.
 32. The method of claim 20, furthercomprising: receiving further a priori information for the nextsignificant bit; and deriving a third output value for a further nextsignificant bit based on the second output value, the further a prioriinformation and a still further decision boundary.
 33. The method ofclaim 32, wherein the receiving and deriving steps are repeated for oneor more additional next significant bits.
 34. The method of claim 20,wherein the received signal has been modulated in accordance withQuadrature Amplitude Modulation (QAM).
 35. A non-transitorycomputer-readable medium comprising, stored thereupon, a computerprogram product comprising program code portions for, when the computerprogram product is run on a computing device: de-mapping a point in aconstellation diagram into a bit sequence, wherein the de-mappingprovides, for each bit of the bit sequence, extrinsic probabilityinformation in the form of an output value, with a sign of the outputvalue indicating a bit value and a magnitude of the output valueindicating probability information in the form of a distance to adecision boundary in the constellation diagram, and wherein thede-mapping comprises: receiving a signal indicative of a constellationpoint, wherein the constellation point represents a bit sequence havinga most significant bit and at least one next significant bit; deriving afirst output value (L₀) for the most significant bit; receiving a prioriinformation (L₀ ^(a priori)) for the most significant bit; and derivinga second output value (L₁) for the next significant bit based on thefirst output value (L₀), the a priori information and a decisionboundary.
 36. A de-mapper apparatus for de-mapping a point in aconstellation diagram into a bit sequence, wherein the de-mappingprovides for each bit of the bit sequence extrinsic probabilityinformation in the form of an output value with a sign of the outputvalue indicating a bit value and a magnitude of the output valueindicating probability information in the form of a distance to adecision boundary in the constellation diagram, the de-mappercomprising: a first interface circuit configured to receive a signalindicative of a constellation point, wherein the constellation pointrepresents a bit sequence having a most significant bit and at least onenext significant bit; a second interface circuitry configured to receivea priori information for the most significant bit; and at least one of aprocessor and circuitry, configured to derive a first output value forthe most significant bit, and to derive a second output value for thenext significant bit based on the first output value, the a prioriinformation and a decision boundary.
 37. A wireless terminal, comprisingthe de-mapper of claim 36 and further comprising a decoder circuitcoupled to the de-mapper and configured to provide the a prioriinformation.
 38. The wireless terminal of claim 37, wherein thede-mapper and the decoder are integrated in an Orthogonal FrequencyDivision Multiplex (OFDM) receiver of the wireless terminal.